Nickel-based superalloy having very high resistance to hot-corrosion for monocrystalline blades of industrial turbines

ABSTRACT

Nickel-based superalloy, suitable for monocrystalline solidification, having the following composition by weight:  
                                           Co:    4.75 to 5.25%         Cr:    15.5 to 16.5%         Mo:    0.8 to 1.2%         W:    3.75 to 4.25%         Al:    3.75 to 4.25%         Ti:    1.75 to 2.25%         Ta:    4.75 to 5.25%         C:   0.006 to 0.04%         B:   ≦0.01%         Zr:   ≦0.01%         Hf:     ≦1%         Nb:     ≦1%         Ni and any impurities:   complement to 100%.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.10/460,860, filed Jun. 12, 2003, which is a continuation of U.S.application Ser. No. 09/999,167, filed Nov. 29, 2001, which claimspriority of European Patent Application Number EP00403361, filed Nov.30, 2000.

TECHNICAL FIELD

The invention relates to a nickel-based superalloy which is adapted tothe manufacture of fixed and movable monocrystalline blades ofindustrial gas turbines by directional solidification.

BACKGROUND OF THE INVENTION

Nickel-based superalloys are the most high-performance materials usedtoday in the manufacture of movable and fixed blades of industrial gasturbines. The two principal features required until now of these alloysfor those specific applications have been good resistance to creep attemperatures of up to 850° C. and very good resistance to hot-corrosion.Some reference alloys currently used in this field are designated IN738,IN939 and IN792.

Blades manufactured using those reference alloys are produced byconventional casting using the lost-wax process and have apolycrystalline structure, that is to say, they are constituted by thejuxtaposition of crystals which are orientated in a random mannerrelative to each other and which are called grains. Those grains arethemselves constituted by an austenitic gamma (γ) matrix based onnickel, in which hardening particles of the gamma prime (γ′) phase aredispersed whose base is the intermetallic compound Ni₃A1. This specificstructure of the grains gives those alloys a high level of creepresistance up to temperatures in the order of 850° C., which ensures thelongevity of the blades, for which service lives of from 50,000 to100,000 hours are generally sought. The chemical composition of alloysIN939, IN738 and IN792 has further been determined to give themexcellent resistance to the combustion gas environment, in particular inrespect of hot-corrosion, a phenomenon which is particularly aggressivein the case of industrial gas turbines. Significant additions of chrome,typically of from 12 to 22% by weight, are thus necessary to give thosealloys the necessary resistance to hot-corrosion for the applicationsconcerned. From the point of view of resistance to creep, the order ofthe alloys is: IN939<IN738<IN792. From the point of view of resistanceto hot-corrosion, the order is the reverse, that is: IN792<IN738<IN939.

In order to improve the performance of industrial gas turbines in termsof output and consumption, one method consists in increasing thetemperature of the gases at the turbine inlet. This consequently makesit necessary to be able to provide alloys for turbine blades which cantolerate operating temperatures which are higher and higher, whilstretaining the same mechanical features, in particular in terms of creep,in order to be able to achieve the same service lives.

The same type of problem has been posed in the past in the case of gasturbines for turbo-jets and turbo-engines for aeronautical applications.In this case, the selected solution consisted in changing from blades,known as polycrystalline blades, which are produced by conventionalcasting to blades, known as monocrystalline blades, that is to say,which are constituted by a single metallurgical grain.

Those monocrystalline blades are manufactured by directionalsolidification with lost-wax casting. The elimination of grainboundaries, which are preferential locations for creep deformation atelevated temperature, has allowed the performance of nickel-basedsuperalloys to be increased spectacularly. Furthermore, the process ofmonocrystalline solidification allows the preferred orientation ofgrowth of the monocrystalline component to be selected and, that manner,the orientation <001> which is optimum from the point of view ofresistance to creep and thermal fatigue to be chosen, those two types ofmechanical stress being the most disadvantageous for turbine blades.

However, the chemical superalloy compounds developed for monocrystallineturbine blades for aeronautical applications are not suitable for bladesfor terrestrial or marine applications, known as industrialapplications. Those alloys are determined in order to promote theirmechanical resistance up to temperatures greater than 1100° C., and thisto the detriment of their resistance to hot-corrosion. In that manner,the concentration of chrome of the superalloys for aeronauticalmonocrystalline turbine blades is generally less than 8% by weight,which allows volume fractions of the γ′ phase in the order of 70% to beachieved, which levels are advantageous for resistance to creep atelevated temperature.

A nickel-based superalloy which is rich in chrome and which is suitablefor the monocrystalline solidification of components of industrial gasturbines is known by the designation SC16 and is described in FR 2 643085 A. Its concentration of chrome is equivalent to 16% by weight. Thefeatures concerning the creep resistance of alloy SC16 are such that thealloy provides, relative to the polycrystalline reference alloy IN738,an increase in operating temperature ranging from approximately 30° C.(830° C. instead of 800° C.) to approximately 50° C. (950° C. instead of900° C.). Comparative tests for cyclical corrosion at 850° C. in air atatmospheric pressure with Na₂SO₄ contamination showed that theresistance to hot-corrosion of alloy SC16 was at least equivalent tothat of the reference polycrystalline alloy IN738.

Hot-corrosion tests have been carried out on alloy SC16 by themanufacturers of industrial turbines on their own test benches. In verysevere environments, which are representative of extreme operatingconditions, it has been shown that the resistance to hot-corrosion ofthat alloy remained inferior to that of alloy IN738.

Furthermore, the increasing demand from those manufacturers for anincrease in the operating temperature of gas turbines gives rise to theneed for superalloys for blades to have a resistance to creep which isincreased still further.

SUMMARY OF THE INVENTION

The problem addressed by the invention is to provide a nickel-basedsuperalloy having a resistance to hot-corrosion in the aggressivecombustion gas environment of industrial gas turbines which is at leastequivalent to that of reference polycrystalline superalloy IN738, andhaving a resistance to creep which is greater than or equal to that ofreference alloy IN792 within a temperature range of up to 950° C.

This superalloy must in particular be suitable for manufacture of fixedand movable monocrystalline blades having large dimensions (up toseveral tens of centimeters in height) of industrial gas turbines bydirectional solidification.

Furthermore, this superalloy must demonstrate good micro-structuralstability in respect of the precipitation of fragile intermetallicphases which are rich in chrome when maintained for sustained periods atelevated temperature.

More specifically, an alloy compound is sought which ensures:

-   -   optimized resistance to hot-corrosion, in any case at least        equivalent to that of reference polycrystalline superalloy        IN738, and this in an environment which is representative of        that for combustion gases of industrial turbines;    -   a maximum volume fraction of hardening precipitates of the γ′        phase in order to promote resistance to creep at elevated        temperature;    -   resistance to creep up to 950° C. which is at least equivalent        to that of reference polycrystalline alloy IN792;    -   a tendency to homogeneity by completely placing in solution        particles of the γ′ phase, including the γ/γ′ eutectic phases;    -   the absence of precipitation of fragile intermetallic phases        which are rich in chrome, starting from the γ matrix, when        maintained for sustained periods at elevated temperature;    -   a density which is less than 8.4 g·cm⁻³ in order to minimize the        mass of the monocrystalline blades and, consequently, to limit        the centrifugal stress acting on the blades and on the turbine        disc to which they are fixed;    -   a good tendency to monocrystalline solidification of turbine        blades whose height can reach several tens of centimeters and        the mass several kilograms.

The superalloy according to the invention, which is suitable formonocrystalline solidification, has the following composition by weight:Co:  4.75 to 5.25% Cr:  15.5 to 16.5% Mo:  0.8 to 1.2% W:  3.75 to 4.25%Al:  3.75 to 4.25% Ti:  1.75 to 2.25% Ta:  4.75 to 5.25% C: 0.006 to0.04% B: ≦0.01% Zr: ≦0.01% Hf:   ≦1% Nb:   ≦1% Ni and any impurities:complement to 100%.

The alloy according to the invention is an excellent compromise betweenresistance to creep and resistance to hot-corrosion. It is for themanufacture of monocrystalline components, that is to say, componentswhich comprise a single metallurgical grain. This specific structure isobtained, for example, by means of a conventional directionalsolidification process at a thermal gradient, using a helical orchicane-like device for selecting a grain, or a monocrystal nucleus.

The invention also relates to an industrial turbine blade which isproduced by monocrystalline solidification of the above superalloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be set forth ingreater detail in the description below with reference to the appendeddrawings.

FIGS. 1 to 4 are graphs illustrating the properties of differentsuperalloys.

DETAILED DESCRIPTION

An alloy according to the invention designated SCA425 has been producedwith reference to the nominal composition listed in Table I. In thistable, the nominal concentrations of major elements of reference alloysIN939, IN738, IN792 and SC16 are also listed. TABLE I Concentrations byweight of major elements (%) Alloy Ni Co Cr Mo W Al Ti Ta Nb IN939 Base19 22.5 — 2 1.9 3.7 1.4 1   IN738 Base 8.5 16 1.7 2.6 3.4 3.4 1.7 0.9IN792 Base 9 12.4 1.9 3.8 3.1 4.5 3.9 — SC16 Base — 16 3 — 35 3.5 3.5 —SCA425 Base 5 16 1 4 4 2 5 —

Chrome has an advantageous and dominant effect on the resistance tohot-corrosion of nickel-based superalloys. Thus, tests have shown that aconcentration in the order of 16% by weight was necessary in the alloyof the invention in order to obtain resistance to hot-corrosion that isequivalent to that of reference alloy IN738 under the conditions forhot-corrosion tests described below, which conditions are representativeof the environment created by combustion gases of some industrialturbines. Chrome also contributes to the hardening of the γ matrix inwhich this element is preferentially distributed.

Molybdenum greatly hardens the γ matrix in which the element ispreferentially distributed. The quantity of molybdenum which can beintroduced to the alloy is limited, however, because the element has adisadvantageous effect on the resistance to hot-corrosion ofnickel-based superalloys. A concentration in the order of 1% by weightin the alloy of the invention is not detrimental to the corrosionresistance and contributes significantly to its hardening.

Cobalt also contributes to the hardening in the form of a solid solutionof the γ matrix. The concentration of cobalt has an effect on thedissolution temperature of the γ′ hardening phase (γ′ solvustemperature). Thus, it is advantageous to increase the concentration ofcobalt in order to decrease the solvus temperature of the γ′ phase andto facilitate the homogenizing of the alloy by means of heat treatmentwithout any risk of causing melting to start. Furthermore, it can alsobe advantageous to reduce the concentration of cobalt in order toincrease the solvus temperature of the γ′ phase and to benefit in thatmanner from greater stability of the γ′ phase at elevated temperature,which promotes resistance to creep. A concentration in the order of 5%by weight of cobalt in the alloy of the invention leads to an optimumcompromise between a good capacity for homogenizing and good resistanceto creep.

Tungsten, whose concentration is in the order of 4% by weight in thealloy of the invention, is distributed in a substantially equal mannerbetween the γ and γ′ phases and, in that manner, contributes to therespective hardening processes thereof. Its concentration in the alloyis, however, limited because the element is heavy and has a negativeeffect on the resistance to hot-corrosion.

The concentration of aluminum is in the order of 4% by weight in thealloy of the invention. The presence of the element causes theprecipitation of the γ′ hardening phase. Aluminum also promotesresistance to oxidation. The elements titanium and tantalum are added tothe alloy of the invention in order to reinforce the γ′ phase in whichthey are substituted for the element aluminum. The respectiveconcentrations of those two elements in the alloy of the invention arein the order of 2% by weight for titanium and 5% by weight for tantalum.Under the conditions for hot-corrosion tests described below,corresponding to the intended application, tests showed that thepresence of tantalum was more favorable to the resistance tohot-corrosion than was the case with titanium. However, tantalum isheavier than titanium, which is disadvantageous in respect of thedensity of the alloy. The total of the concentrations of tantalum,titanium and aluminum roughly determines the volume fraction of the γ′hardening phase. The concentrations of those three elements have beenadjusted in order to optimize the volume fraction of the γ′ phase, whilekeeping the γ and γ′ phases stable when maintained for long periods atelevated temperature, and taking into consideration the fact that theconcentration of chrome has been fixed at approximately 16% by weight inorder to achieve the desired resistance to corrosion.

Alloy SCA425 has been produced in the form of monocrystals havingorientation <001>. The density of that alloy has been measured and foundto be equal to 8.36 g·cm³.

After directional solidification, the alloy is substantially constitutedby two phases: the austenitic γ matrix, which is a solid nickel-basedsolution, and the γ′ phase, which is an intermetallic compound whosebasic formula is Ni₃Al and which precipitates mainly within the γ matrixin the form of fine particles measuring less than 1 micrometer duringcooling to the solid state. Contrary to what is generally found inmonocrystalline superalloys for turbine blades, alloy SCA425 does notcontain any interdentritic solid particles of the γ′ phase resultingfrom a eutectic transformation of the residual liquid oncesolidification has ended.

Alloy SCA425 underwent homogenizing heat treatment at a temperature of1285° C. for 3 hours with cooling in air. This temperature is higherthan the solvus temperature of the γ′ phase (dissolution temperature ofthe precipitates of the γ′ phase), which is 1198° C., and less than thesolidus temperature, which is 1300° C. The treatment is intended todissolve all of the precipitates of the γ′ phase, whose distribution ofsizes is very wide in the coarse state of directional solidification,and to reduce the chemical heterogeneities which are associated with thedendritic solidification structure.

The interval between the γ′ solvus temperature of the alloy SCA425 andits solidus temperature is very large, which allows ready application ofthe homogenizing treatment without any risk of melting and with thecertainty of obtaining a homogeneous microstructure which allowsoptimized resistance to creep.

The cooling which follows the homogenizing treatment described above wascarried out by hardening in air. In practice, the rate of this coolingmust be so high that the size of the particles precipitated during thecooling operation is less than 500 nm.

The homogenizing heat treatment procedure which has just been describedis an example which allows the intended result to be achieved, that isto say, a homogeneous distribution of fine particles of the γ′ phasewhose size is no greater than 500 nm. This does not exclude thepossibility of obtaining a similar result by using a different treatmenttemperature provided that the temperature lies within the rangeseparating the γ′ solvus temperature and the solidus temperature.

Alloy SCA425 was tested after undergoing a homogenizing treatment asdescribed above, then two annealing treatments which allow the size andthe volume fraction of the precipitates of the γ′ phase to bestabilized. A first annealing treatment consisted in heating the alloyto 1100° C. for 4 hours with cooling in air, which leads tostabilization of the size of the precipitates of the γ′ phase. A secondannealing treatment at 850° C. for 24 hours, followed by cooling in air,allows the volume fraction of the γ′ phase to be optimized. This volumefraction of the γ′ phase is estimated at 50% in alloy SCA425. Once allof the heat treatments are completed, the majority of the γ′ phase hasbeen precipitated in the form of cuboid particles whose size is between200 and 500 nm. A small fraction of fine particles of the phase γ′ whosesize does not exceed 50 nm is present between the large precipitates.

Hot-corrosion tests were carried out at different temperatures on alloySCA425 using the following procedure: samples are partially immersed ina container containing a mixture of combustion residues whosecomposition by weight is as follows: 4.3% Na₂SO₄+22.7% CaSO₄+22.3%Fe₂O₃+20.6% ZnSO₄+10.4% K₂SO₄+2.8% MgO+6.5% Al₂O₃+10.4% SiO₂. A mixtureof air+0.15% SO₂ by volume passes through the mixture of combustionresidues at a rate of 6 liters per hour. The mixture of combustionresidues is renewed every 500 hours. This environment is representativeof the very aggressive environment of combustion gases for someindustrial turbines. For comparison purposes, samples of alloys IN738,IN939, IN792 and SC16 were tested at the same time.

The samples were cut into sections and the depth of metal destroyed bythe corrosion phenomenon was measured. The graphs in FIGS. 1 to 3 showthe mean depths of penetration of the corrosion for the different alloysat 700° C., 800° C. and 850° C., respectively, as a function of the testduration. The resistance to corrosion is even better since the depth ofpenetration is low. At 700° C. and 800° C., the alloy SCA425demonstrates a resistance to corrosion equivalent to that of alloy IN738and better than that of alloy SC16. At 850° C., the resistance tocorrosion of alloy SCA425 is comparable to that of reference alloysIN738 and IN939.

Tests for creep under tensile stress were carried out on machined testpieces in monocrystalline bars of orientation <001>. The bars werehomogenized beforehand then annealed according to the proceduresdescribed above. Values for rupture times obtained at 750, 850 and 950°C. for different levels of stress applied are listed in Table II. TABLEII Service lives with creep of alloy SCA425 Temperature (° C.) Stress(MPa) Rupture time (h) 750 650  216/321.1 750 575  984 850 400  201/276850 300 2121/2945/3220 850 250 6161 950 250  73/76 950 200  261/291 950180  578 950 160 1098 950 140 2109 950 120 3872

The graph in FIG. 4 allows a comparison of the rupture times with creepobtained for alloys SCA425, IN792 and SC16. The stress applied isplotted on the abscissa. The value of the Larson-Miller parameter ismarked on the ordinate. This parameter is given by the formulaP=T(20+log t)_(x 10)-3, where T is the creep temperature in Kelvin and tis the rupture time in hours. This graph shows that the creep resistanceof alloy SCA425 is at least equivalent to that of alloy IN792, which isthe stipulated objective, and greater than that of reference alloy SC16.

The inspection of the microstructure of the test pieces of alloy SCA425at the end of the creep tests demonstrated the absence of precipitationof fragile intermetallic particles which are rich in chrome and whichare capable of appearing when maintained for sustained periods atelevated temperature in nickel-based superalloys where the γ matrix isover-saturated with additive elements.

Manufacturing tests on monocrystalline components of super-alloy SCA425demonstrated that it was possible to cast a large range of componentswhose mass can range from a few grams to more than 10 kg, with variouslevels of complexity. The growth of components according to thecrystallo-graphic orientation <001> is promoted and dominant and thepresence of grains that are orientated in a random manner is minimized.The liquid metal is stable in the sense that it does not react with thematerials commonly used in the manufacture of moulds. The phenomenon ofrecrystallisation which can occur during homogenizing treatment atelevated temperature is absent in the case of alloy SCA425.

1. A nickel-based superalloy, suitable for monocrystallinesolidification, characterized in that its composition by weight is asfollows: Co:  4.75 to 5.25% Cr:  15.5 to 16.5% Mo:  0.8 to 1.2% W:  3.75to 4.25% Al:  3.75 to 4.25% Ti:  1.75 to 2.25% Ta:  4.75 to 5.25% C:0.006 to 0.04% B: ≦0.01% Zr: ≦0.01% Hf:   ≦1% Nb:   ≦1% Ni and anyimpurities: complement to 100%.


2. Industrial turbine blade produced by monocrystalline solidificationof a superalloy according to claim 1.